This invention relates generally to the deposition of heat or materials onto a substrate and, more particularly, to spatial stabilization of flames for combustion synthesis.
It is well known to those skilled in the art that certain flow configurations have important similarity properties that render their analysis one-dimensional. Included in this set is stagnation flow. Given that a uniform velocity, uniform temperature and uniform composition inlet flow issues from a manifold or nozzle a fixed distance above a parallel fixed solid surface which is at uniform temperature, it can be shown that the heat and mass flux to the solid surface will be everywhere uniform regardless of the radial extent of the system. In addition, the gas phase species and temperature profiles are independent of radius. A more complete discussion of stagnation flow is contained in co-pending application Ser. No. 08/302,155, incorporated herein by reference. The inherent radial uniformity of a stagnation flow geometry provides an important means for achieving uniform species and heat fluxes to large surface areas. This technology has application to materials synthesis processes, such as chemical vapor deposition for the fabrication of semiconductors and flame synthesis of diamond films all of which require very highly uniform film growth over relatively large areas so that many identical devices can be cut from a single large wafer.
A stagnation flow reactor useful for the flame synthesis of a diamond film is shown in FIG. 1. A reactive gas mixture 105 is caused to flow through nozzle 110 and the gas mixture is ignited subsequent to its passage through nozzle 110 to form a flame 140 which impinges onto substrate 120. This reactor is designed to yield a flat flame reaction zone such that uniform fluxes of reactive species and heat necessary for the growth of the diamond film are supplied to substrate surface 120. In order to obtain the desired diamond film growth rates it is necessary to strain or press the leading edge or front of flame 140 as close as possible to the surface of substrate 120. This can be accomplished by decreasing the separation distance d between nozzle surface 110 and substrate surface 120. In order for the film grown on substrate 120 to be of uniform thickness across the surface of substrate 120 it is necessary that nozzle 110 be parallel to substrate 120. It will be appreciated that as the distance d is decreased to values desirable for an acceptable growth rate for the flame synthesis of diamond, the rate of strain, defined as the gradient of the velocity profile at the flame front, is near the extinction limit where the flame cannot be sustained. In this regime the flame displays certain angular assymmetries that are manifest in a series of azimuthal structures 210, as depicted in FIG. 2. Here the n=3 mode number azimuthal structure is shown for a highly strained flame. The flame is very unstable during these low mode number instabilities and flame extinction typically occurs within a time-scale of seconds. What is needed is a method for eliminating or reliably controlling the low mode number instabilities that appear under conditions of highly strained flame synthesis and for providing long term stability (on the order of hours) for the flame front for use in combustion or flame synthesis applications, particularly synthesis of diamond.